Michel Olagnon

Phylogenetic characterization of the bacterial assemblage associated with mucous secretions of the hydrothermal vent polychaete Paralvinella palmiformis.

As part of an ongoing examination of microbial diversity associated with hydrothermal vent polychaetes of the family Alvinellidae, we undertook a culture-independent molecular analysis of the bacterial assemblage associated with mucous secretions of the Northeastern Pacific vent polychaete Paralvinella palmiformis. Using a molecular 16S rDNA-based phylogenetic approach, clone libraries were constructed from two samples collected from active sulfide edifices in two hydrothermal vent fields. In both cases, clone libraries were largely dominated by epsilon-Proteobacteria. Phylotypes belonging to the Cytophaga-Flavobacteria and to the Verrucomicrobia were also largely represented within the libraries. The remaining sequences were related to the taxonomic groups Fusobacteria, Green non-sulfur bacteria, Firmicutes, gamma- and delta-Proteobacteria. To our knowledge, this is the first report of the presence of Verrucomicrobia, Fusobacteria and green non-sulfur bacteria on hydrothermal edifices. The potential functions of the detected bacteria are discussed in terms of productivity, recycling of organic matter and detoxification within the P. palmiformis microhabitat.

Advances in search and rescue at sea

A topical collection on “Advances in Search and Rescue at Sea” has appeared in recent issues of Ocean Dynamics following the latest in a series of workshops on “Technologies for Search and Rescue and other Emergency Marine Operations” (2004, 2006, 2008, and 2011), hosted by IFREMER in Brest, France. Here, we give a brief overview of the history of search and rescue at sea before we summarize the main results of the papers that have appeared in the topical collection.

Practical computation of statistical properties of rainflow counts

Abstract Some mathematical properties of the rainflow counting method were established by Rychlik. They enabled him to propose a new algorithm for rainflow counting. Implementations of this algorithm for signal analysis would show poor speed performance, but it is of utmost interest as it can be used abstractly on the statistical properties of the input sequence. The practical implementation of such derivation is discussed here, and probability density functions (PDFs) for rainflow cycles between level j and level k are derived in the simple case when peaks and troughs are chosen from known PDFs in a completely random succession. Algorithms are provided in a common practice case when the sequence is described as a Markovian process with given transition probability matrix. Sequences for which the statistical properties are inferred from the power spectral densities are also discussed. Application is made to some standard sequences from the offshore and automotive industry.

West Africa Swell Spectral Shapes

Wave spectra measured at sites off West Africa are dominated by the constant presence of one or several swell wave systems. The West Africa Swell Project (WASP JIP) was carried out to propose and assess parametric models for the shap es of the swell components. Bias, variability, and dispersionof estimates are affected by the length/stationarity comprom ise of the record lengths and the window-tapering used to reduce th eir variability. In particular, shapes with sharp angles are st rongly smoothed, for instance a triangular peak would appear round and reduced by 15 to 25% with rectangular or Tuckey windowing. Models that consider each wave system individually, an d an arbitrary number of those, were preferred to global ones. Pa rtitioning of directional spectra is thus a prerequisite, an d needs to be tuned taking account some prior knowledge of the swell characteristics. Triangular, log-normal, Gaussian and Gl ennJonswap shapes were considered. Sampling variability make s it difficult to distinguish between those shapes as far as swell s are concerned. The models also indicate that the width of the spe ctrum in frequency should be inversely proportional to the pe ak frequency. Directional spreading width shows a similar tre nd.

Evaluating height–length joint distributions for the crests of ocean waves

The wavelength is an important quantity in a number of ocean-engineering applications, e.g. for design of structures that have dimensions of the same order of magnitude as the wavelength. Most often however, the description of the wave climate is given by a spectrum. We present a methodology based on a representation of a sea state in the frequency domain which returns the joint distribution of crest amplitudes and wavelengths. The method is validated with experimental empirical data, obtained by application of a technique based on Fourier transformation combined with use of the dispersion relation. The algorithm allows for investigation of the influence of spectral shape and directionality on the joint distribution; examples are given in the paper.

THE WEST AFRICA SWELL PROJECT (WASP)

The responses of floating systems are sensitive to the detailed shape of the swell portion of the wave spectrum. Knowledge of swell is particularly important for sites off West Africa. The West Africa Swell Project (WASP JIP) was formed to analyze the available data on West African swell. Measurements were obtained from Shell, Ifremer, Chevron and Marathon. Hindcast data came from Oceanweather and the NOAA Wavewatch model. Sites ranging from Nigeria to Namibia were considered.Modeling the dispersion of swell over long distances indicates that the resulting power spectrum will have a triangular or lognormal shape. Sampling variability makes it difficult to distinguish between those shapes or Jonswap or Gaussian forms, but lognormal spectra generally provided good fits. The models also indicate that the width of the spectrum in both frequency and direction should be inversely related to the peak frequency. Fits to the measurements established detailed relationships for each location.Calculating the response of single degree of freedom oscillators to the measured and hindcast spectra produces response spectra which give the maximum response as a function of natural period and damping. Extreme values of system response can be calculated from the response spectra. The largest responses come from uni-modal spectra, and design spectra can be estimated by inflating them.Copyright

Wave Spectra Partitioning and Identification of Wind Sea and Swell Events

Wave spectra often exhibit several peaks due to the coexistence of wind sea generated by local wind conditions and swells originating from distant weather systems. For an accurate description of sea states, those spectra can be partitioned and the partitions can be interpreted physically as representing independent wave systems. Furthermore, partitions can be associated in time to track the evolutions of wave systems originating from the same meteorological event (storm, depression, etc).This paper proposes a new method to identify temporal sequences of wave systems parameters, consistent with respect to the meteorological events that are the sources of the phenomena. This method, as many conventional partitioning ones, is based on the watershed algorithm which is however directly applied in our case to the whole time-history of wave spectra. Using appropriate criteria, the identified events are classified into swell or wind sea events. Field data from West Africa are used to illustrate the method and the results are compared with those of conventional software. Hindcast data from the Iroise Sea are also used to validate the proposed method and the results show a good identification of wave systems events with a good correlation between wind sea events and wind characteristics.Copyright

RESPONSE SENSITIVITY TO SWELL SPECTRA OFF WEST AFRICA

Both hindcast and measured wave data were used to establish various descriptions of the swell spectra off West Africa in the West Africa Swell JIP (WASP). The effect on system responses of the various data types and swell spectral descriptions has been estimated.The response of single degree of freedom oscillators to wave spectra can be used to produce maximum responses as a function of natural period and damping, analogous to the approach used in earthquake engineering. The results can be used to gain insight into the sensitivity of floating systems to the swell characteristics of the location and also to the differences between hindcast and measured data sets. Extreme values of system response can be calculated and used to estimate design spectra, by scaling the measured or hindcast spectrum that produced the largest response.The sensitivity of the responses of floating systems to various simplifications of the hindcast spectra was evaluated by calculating the responses of an FPSO and CALM buoy for a location off Nigeria and southern Angola. It was found that there is little loss of accuracy if the hindcast spectra are decomposed into a maximum of two partitions, one of which may be a wind-sea, and the swell partitions are described with a lognormal function, the wind-sea partition with a JONSWAP-Glenn frequency function, and the directional spreading with a wrapped normal function, together with parametric forms for the widths of the lognormal and wrapped normal functions. As a consequence it is concluded that the parametric forms for the swell frequency and direction spreading can be used in design.Copyright

Parametric Rainflow Fatigue Damage for Some Power Spectra of Common Use

Rainflow counting is widely accepted as the method that is most suited to analysis of fatigue damage of materials submitted to irregular loading. Formulas such as the Wirsching-Light and the Dirlik one allow to take into account spectral shape and bandwidth in an empirical or semi-empirical manner to obtain a best estimate damage reduction of the rainflow counting with respect to the narrow-band approximation. However, if one considers parametric shape families of common use for the spectra, a more straightforward way is to make damage depend on the shape parameter of the family rather than on the spectral moments. We provide here such semi-empirical parametric formulas for the Jonswap, Wallops, Triangle and Power-tail families. In addition, the ICA formula allows us to extend the above formulas to the well-known bimodal spectral shape proposed by Ochi-Hubble.Copyright